1. Field of the Invention
This invention relates to a production technology of semiconductor devices both on an SOI (Semiconductor On Insulator) structure portion and on a semiconductor substrate not having the SOI structure portion of a partial SOI substrate. More particularly, this invention relates to a method of producing semiconductor devices which includes a step of making the thickness of an insulation film for device isolation different between an SOI structure portion and an exposed portion of a semiconductor substrate not having the SOI structure portion, when the insulation film (oxide film) is used for device isolation in order to electrically isolate devices from one another.
2. Description of the Relate Art
The following methods are possible measures for forming device isolation regions both in SOI structure portion comprising an insulation layer and a semiconductor layer, and in an exposed substrate portion of a semiconductor substrate (hereinafter referred to as the "bulk substrate portion") in a substrate comprising the SOI structure portion and the bulk substrate portion (which substrate will be hereinafter referred to as a "partial SOI substrate"). The first method is the one that simultaneously forms both of them. When a production method is directed to fabricate complete depletion type transistors in the SOI structure portion by forming a semiconductor layer to a thin film in the SOI structure portion, the thickness of a device isolation oxide film of a field transistor of the bulk substrate portion becomes small if this device isolation oxide film is fabricated under the formation condition of the device isolation region of the SOI structure portion. Therefore, the insulation resistance of the field transistor cannot be secured sufficiently.
When the formation of the device isolation region is conducted under the formation condition of the device isolation oxide film of the bulk substrate portion, on the other hand, oxidation becomes excessive in the SOI structure portion. In consequence, a semiconductor layer (Si layer) at the ends of an active region of the SOI structure portion is oxidized, too. Thus the film thickness of the semiconductor layer becomes locally thin (i.e. the semiconductor layer at the end of the active region is thin) and the device isolation oxide film creates a stress with the result that transistor characteristics get deteriorated.
When the thickness of the device isolation oxide film of the SOI structure portion is different from the thickness of the device isolation oxide film of the bulk substrate portion, the device isolation region and a well region are generally formed by the following second method (see FIGS. 5(a) to 7(c)).
FIG. 5(a) shows a partial SOI substrate. In the drawing, reference numeral 10 denotes a semiconductor layer. Reference numeral 20 denotes an insulation layer and reference numeral 30 denotes a semiconductor substrate. The following method is known as a fabrication method of this partial SOI substrate, by way of example. An oxygen ion, or the like, is implanted into a suitable depth of a semiconductor substrate (silicon substrate), and a silicon oxide layer (buried insulation layer) is formed in the silicon substrate. As a result of this process step, the silicon layer is formed on the silicon oxide layer. Next, the silicon oxide layer and the silicon layer on the former are selectively etched away, giving the structure shown in FIG. 5(a). Alternatively, the partial SOI substrate having the structure shown in FIG. 5(a) can be acquired by laminating selectively and serially the insulation layer (silicon oxide film) and the semiconductor layer (silicon layer) on a semiconductor substrate (silicon substrate).
Next, as shown in FIG. 5(b), a pad oxide film 40 and a nitride film 50 are deposited over the entire surface of the substrate by thermal oxidation and by vacuum CVD, respectively.
A photolithography step (resist patterning step), an etching step, and an oxidation step for forming a device isolation region, are carried out for forming an active region and a device isolation region in the SOI structure portion as shown in FIGS. 5(c) to (e). Incidentally, reference numeral 60' in FIG. 5(c) denotes a patterned photoresist mask, and reference numeral 70-1 in FIG. 5(e) denotes a device isolation oxide film in the SOI structure portion.
Next, a nitride film 80 is deposited by vacuum CVD as shown in FIG. 6(a). A photolithography step (resist patterning step), an etching step and an oxidation step for forming a device isolation region are carried out for forming an active region and a device isolation region subsequently for the bulk substrate portion as shown in FIGS. 6(b) to (d). Incidentally, reference numeral 60'-1 in FIG. 6(b) denotes a patterned resist mask, and reference numeral 70-3 in FIG. 6(d) denotes a device isolation oxide film in the bulk substrate portion. Thereafter, the nitride films 50 and 80 are etched away. Resist masks 60'-2 and 60'-3 for forming the well are formed in the SOI structure portion and the bulk substrate portion, respectively, as shown in FIGS. 7(a) to (c). Next, an impurity ion is implanted (90 and 100) into each region, and annealing treatment is carried out for activation, thereby giving the well.
In the production method of a semiconductor device using a partial SOI substrate, the second method described above is generally used in order to acquire reliably the desired device characteristics. According to this method, however, the photolithography step, the etching step and the oxidation step for forming the device isolation region must be carried out for both SOI structural portion and bulk substrate portion. Furthermore, the photolithography step for forming the wells must also be carried out for the SOI structure portion and the bulk substrate portion (see FIGS. 5(c), 6(b), 7(b) and 7(c)). In other words, the photolithography step must be conducted four times in total. The number of the photolithography steps must be decreased in order to decrease the production time and the production cost.